Prior to the present invention as shown by Heath, et al. U.S. Pat. Nos. 3,879,428 and 3,957,862, assigned to the same assignee as the present invention, aromatic bis(ether anhydrides) (hereinafter referred to as "bisanhydrides") of the general formula: ##STR1## were made by a multi-step procedure involving the base hydrolysis of an aromatic bis(ether N-organo substituted phthalimide) of the formula: ##STR2## A more specific compound of which is 2,2-bis[4-(3,4-dicarboxy)phenyl]propane bis-N-methylimide which has the formula: ##STR3## where R is a divalent aromatic radical having from 6-30 carbon atoms and R.sup.1 is a monovalent organic radical selected from the class consisting of C.sub.(1-8) alkyl radicals, and organic radicals, having from 6-20 carbon atoms, for example, aromatic hydrocarbon radicals and halogenated derivatives thereof. This procedure produced a tetra-acid salt which was thereafter acidified to the tetra-acid followed by the dehydration of the tetra-acid to produce the aromatic bis(ether anhydride) of formula I.
Although the procedure of Heath et al. provides a valuable route to both the aromatic bis(ether phthalic acids) and aromatic bis(ether phthalic anhydrides) it requires the base hydrolysis of the aromatic bis(ether N-organo substituted phthalimide of formula II and the conversion of the resulting salt to the tetra-acid, followed by the dehydration of the tetra-acid. In addition to requiring a variety of steps to convert the bisimide to a bisanhydride, inorganic salts are generated causing disposal problems. Efforts, are, therefore, being directed to providing a more simplified procedure for making the bisanhydride of formula I, or its tetra-acid precursor.
Markezich, et al. U.S. Pat. No. 4,128,574 discloses an imide-anhydride exchange reaction resulting in the production of organic polycarboxylic acids, anhydrides thereof, or organic imides. For example, in particular instances, a bisimide of formula IIA, is heated with phthalic anhydride in the presence of water to effect an exchange between the aforementioned bisimide and the phthalic anhydride to provide the corresponding tetra-acid or anhydride thereof.
Although the Markezich, et al. method eliminates many of the disadvantages of the prior art, such as the formation of inorganic salts or the requirement of a multi-step procedure, Markezich, et al. is essentially a batch method. The recovery of a tetra-acid or bisanhydride at a satisfactory yield, 80% or higher concentration, requires several heating and stripping cycles. It is also difficult to achieve substantial conversion of the bisimide to the tetra-acid or the bisanhydride without resort to the recycling of excessive amounts of phthalic acid or phthalic anhydride. Based on the nature of the exchange between the bisimide and phthalic acid or phthalic anhydride, optimum conversion cannot be realized unless the N-organo phthalimide of the general formula: ##STR4## where R.sup.1 is as previously defined, which is also formed in the reaction, is separated from the mixture.
Webb, in U.S. Pat. No. 4,116,980, showed that optimum conversion of the bisimide to the tetra-acid or dianhydride thereof, can be achieved based on the imide-anhydride exchange in the presence of water, as shown by the following: EQU A+B.revreaction.A'+B'
where A and A' are imides and B and B' are anhydrides, if the A' imide is selectively removed from the reaction during the exchange. For example, in the above equation, A can be a bisimide, B can be a phthalic acid, B' can be a bis-anhydride or tetra-acid and A' can be an organo phthalimide. Webb achieved these results by venting a portion of the vapor phase of the reaction mixture consisting of a liquid phase and a vapor phase during the exchange. The vapor phase consisted essentially of water and N-organo phthalimides with very little phthalic acid so that by continuously venting the vapor phase during the exchange, the reaction is driven to the right. It is, therefore, possible to convert the starting bisimide to the corresponding tetra-acid or bis anhydride without either shutting down the reactor or recycling excessive amounts of phthalic acid.